In general, a steel box girder, a tubular girder, an I-beam girder and the like are used as steel girders applied to bridges. Among these girders, the steel box girder is best in terms of strength and weight.
The steel box girder is a girder for a mold construction method, which can be constructed up to a maximum span of 70 m. The steel box girder is applicable to curved bridges due to high torsional strength, and has a simple process, a short construction period and excellent strength. On the other hand, since a large number of reinforcement materials should be used inside the steel box girder so as to improve strength, the construction costs of the steel box girder increase, and the weight of the steel box girder increases. Further, the steel box girder is weak against vibration and droop due to characteristics of steel materials.
The structure of the steel box girder will be described with reference to FIG. 1. A rectangular box-shaped steel box girder 10 including upper and lower flanges 11 and 12 has a structure in which a plurality of steel reinforcement materials 13 are arranged inside the steel box girder 10 along lateral and longitudinal directions and inner circumference, and the outside of the steel box girder 10 is reinforced with a cross beam 14. The steel box girder 10 is mounted on a bridge pier 1 so as to support a slab 2 that is an upper structure.
As shown in FIG. 2, a tubular girder 20 has a structure in which a plurality of steel reinforcement materials 23 are arranged inside a ‘U’-shaped girder main body 21 along lateral and longitudinal directions and inner circumference, and the outside of the tubular girder 20 is reinforced with a cross beam 24. The tubular girder 20 is mounted on a bridge pier 1 so as to support a slab 2 that is an upper structure.
As shown in FIG. 3, an I-beam girder 30 has a structure in which side walls of an I-beam 31 are reinforced with a plurality of cross beams 34. The I-beam girder 30 is mounted on a bridge pier 1 so as to support a slab 2 that is an upper structure.
In addition to these girders, a PF beam girder is used. The PF beam girder is a steel composite girder that introduces prestress to a concrete portion by reloading a preflexion load, in additional consideration of 10 to 20% of allowable stress of the steel box girder, and then filling the girder with high strength concrete. The PF beam girder is disadvantageous to be applied to curved bridges, and has the problem of a dead load. Hence, the PF beam girder is frequently used in straight bridges up to a maximum span of 50 m or places requiring a low girder height, such as a downtown area and a river. Since the PF beam girder has a low girder height, it is easy to secure a girder under space, and it is advantageous to plan bridge construction. On the other hand, the construction costs of the PF beam girder increase, and it is difficult to mend and reinforce the PF beam girder in the occurrence of cracks.
Problems of the conventional girders described above will be specifically described as follows. That is, the steel box girder and the tubular girder have large scale and large weight, and use expensive steel materials exhibiting strength characteristics as a tension member for a compression member at an upper portion of the girder, which is inefficient. When considering characteristics of steel materials having weakness in terms of compression strength, an excessive number of steel reinforcement materials should be used to secure the compression strength of the upper portion of the girder, and the torsional strength is weak. Therefore, the weight of steel is increased by 40% or more, and an increase in construction cost is caused. Further, it is difficult to apply the steel box girder and the tubular girder as girders having a maximum long span of 70 m or more due to excessive weight of steel as compared with the strength of steel materials.
In the I-beam girder, the girder height (main girder height) should be increased to secure strength, and the structure of the I-beam girder may be unstable due to the weakness of torsional strength. The I-beam girder is efficient because of its sectional characteristics, but it is difficult to apply the I-beam girder as a girder having a long span.
Particularly, in the existing girders, a large number of steel reinforcement materials should be used so that a web connecting between compression and tension sections of the girder provide a great shearing stress. Therefore, the dead load of a structure increases, and economic efficiency is deteriorated due to the excessive use of construction materials.